Headlight Wiring - 12-Volt Lights on a 24-Volt SystemEarly MCI coaches used 24 volt bulbs for all the exterior lights, including the headlamps. During the mid 1980's the 24 volt headlamps were replaced on the MC-9 coaches with 12 volt headlamps, beginning at unit 34901. By wiring the headlights in series, 12-volt lights will operate with a 24-volt system.

The problem with wiring lights in series is if one lamp fails, both lamps go out. To prevent this from happening, a "balance" wire was added. This wire is routed from the 12-volt post on the rear battery to the wire connecting the headlamps.

Depending on which headlight fails, the balance wire acts either as a power supply or ground connection for the remaining lamp. The balance wire has 12 volts of power on it, which supplies power for the headlamp connected to battery ground. For the headlamp with 24 volts attached, the voltage difference between the two wires will allow 12 volts to flow through the lamp illuminating it (24 volts on power side - 12 volts on ground side = 12 volts power flowing through light.)

Service Bulletin #1930 explains the procedure on converting your older MCI coach over to 12-volt headlamps. It also lists all the parts required for this rework. Beginning in April of 1992, the exterior lighting on MCI coaches was changed to operate on a 12-volt system. All coaches from that point on have been using 12-volt exterior lighting, so the above information does not apply to these later models.

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1989 MCI-102 A3DD 6V92 Turbo, AlisonTons of stuff to learn!Started in Cheboygan, Michigan (near the Mackinaw Bridge). Now home is anywhere we parkhttp://dave-amy.com/

MCi also sold, at one time, little 24v to 12v converter boxes, one per side. I have them on my bus, and they don't work well at all. Even with a Hella conversion the lights are dim. Apparently the reason you want 12V headlamps, aside from the obvious, is that the tungsten filament is heavier and therefore more robust. I read that on a headlamp manufacturer page.

To the question - yes, you can double up the lamps and put them in series to run 12v lamps on a 24v system. It works better if you use LED's due to high current from the two 12v lamps. I have actually done that on one of my marker lights, I had to put something in in a hurry so I got two LED marker lights and put them in series. Works great, looks a little ghetto but it's up high so I don't worry about it.

MCi also sold, at one time, little 24v to 12v converter boxes, one per side. I have them on my bus, and they don't work well at all. Even with a Hella conversion the lights are dim. Apparently the reason you want 12V headlamps, aside from the obvious, is that the tungsten filament is heavier and therefore more robust. I read that on a headlamp manufacturer page.

Does this issue with 12 volt bulbs being better than 24 volt bulbs also apply to 24 volt bulbs used in H4 headlights? I used 24 volt bulbs because the wiring from the batteries to the headlights could be #8 instead of #4 if I used 12 volt.

if you need the standard bayonet mount bulbs, I have gotten them from NAPA. However, depending on the counter person, you may have to ask for 28v bulbs. I have seen LED's on Ebay that list the voltage as a range that includes both 12v and 24v.

We use those magnetic roof mount lights for our toad. I replaced the 12v bulbs that came in the fixtures with 24v ones. Works fine.

This is, of course, another issue. I think that there are electronic adaptors for making the transition. The question is whether they will take 24v or not. Some of our people have set up a system that uses relays to both trigger the 12v signal using the buses 24v signal and integrate the brake light. At the time we were going to first use our magnetic lights, we were in the process of moving so I thought the simplest thing to do at least for the short term was to screw an independent light to the top of each magnetic fixture and wire that for the brake. That was some 6 years ago. Since it worked, I never got motivated to change it.

I just did this to our 5B. It had 12 volts available at the rear junction where the tail light connections are. I used four 24 volt relays and Sean's print. Solves both problems, separate turn and brake lights and 24-12 volt.